It's counter-intuitive, but teenagers have a lot more trouble staying awake at the wheel than older people. As we grow older, the mechanism that helps us quickly shut down and fall asleep doesn't work as well, but in the young, it's still quite powerful. That makes teens particularly susceptible to drowsy driving.

"In young people, when sleep pressure becomes high, the sleep switch is working so well," said Charles Czeisler, chief of the division of sleep and circadian disorders at Brigham and Women's hospital in Boston. "Young people are more vulnerable. It will seize control, and people will sail off the side of the road and into a telephone pole."

So after an Army Reserve major was killed a week before his 2002 wedding by a teenaged driver who had fallen asleep at the wheel, Massachusetts turned to sleep scientists like Czeisler for a solution. Realizing that teens don't believe in their own mortality, want to drive as soon as they can and are often sleep-deprived, the state passed a law in 2007 threatening them with one of the worst penalties a new driver could imagine: immediate license suspension for breaking the driving curfew.

Any "junior" driver -- meaning anyone ages 16½ to 17 -- caught driving between 12:30 a.m and 5 a.m. would have his license suspended for 60 days. A second offense would result in 180 days without a license and a third would cost a year without driving privileges. Previously, the penalties had been a maximum $35 fine for a first offense and a $75 to $100 fine for any after that.

The result, Czeisler and a team of researchers reported this week, has been a dramatic reduction in crashes involving 16- and 17-year-old drivers. The overall crash rate for that age group declined 19.1 percent when compared with drivers older than 20 in the five years after the law was implemented, according to their study. The nighttime accident rate declined 28.8 percent more, and the rate of accidents that were fatal or incapacitated someone dropped 39.8 percent more. The study was published in the June issue of the journal Health Affairs.

"Our findings suggest that driving laws that eliminate or effectively deter unsupervised night driving by people younger than eighteen can achieve substantial reductions in motor vehicle crashes," they wrote. "This change is likely driven by mitigating risks posed by sleepiness-related impairment, risk-taking behaviors, and driving alone or driving with young passengers."

The new law also includes driver education programs that demonstrate how serious drowsy driving can be for teens.

"You want to have them understand that staying awake for 24 hours has as much impact as [driving with a blood alcohol of] .10," Czeisler said. In many states, the legal limit for alcohol impairment is .08. Drinking coffee or energy drinks don't really help, he said, because previous research shows that it tends to make people push their drowsiness limits.

Drowsy driving in general may be the next frontier of highway safety, considering the progress made in reducing drunk driving through tougher laws and public campaigns. In surveys, 8 million drivers admit to falling asleep at the wheel at least once a month (emphasis mine) and 56 million admit to driving while drowsy each month, Czeisler said. Sleepiness is blamed for 400,000 to 1 million crashes annually, 6,400 deaths and 55,000 debilitating injuries.

The study results also show that the rate of drowsy driving accidents declined by a much smaller amount among 18- and 19-year-olds, indicating that states might want to consider extending some form of control on that age group, Czeisler said.

"When they become 18, the crash rate just jumps right back up," he said. "...That’s another area ripe for intervention."